Hmmm. Tubes would be my last suggestion. RCA and Mullard come to mind, as well as Amperex and, for some, Telefunken. But I'd aim for other upgrades first.
Top for me, if you haven't done it yet, would be either choke or CCS load of the tubes. You've replaced caps and I don't think you'll see a big change there from upgrades. Volume pot replacement is another promising upgrade if you haven't done it.
But... more importantly, think about your amp and speakers more than Quickie. A bigger room will change everything. I moved one system from my small living room (13' x 14' or so) to the upstairs of my barn (20' x 25'). The entire system sounded great in the living room but anemic in the barn. The main solution in this case was bigger speakers (in my case Magnepan 3.3r but of course lots of other choices exist). You may need a more powerful amp. You may need speakers with a bigger driver to push more air. And you may want a brighter presentation from the tweeters. In short, anything you do with the Quickie is not likely to have nearly as much effect as the amp and speakers.